The invention is based on a method for measuring the flow velocity, particularly in a wind tunnel, in which a pair of light beams which cross each other illuminates a volume to be measured and a Doppler-shifted scattered light scattered at particles moved along with the flow is registered and converted into electric signals and in which the Doppler frequency of the registered scattered light is used as a measure of the flow velocity.
Such a known method is called laser Doppler anemometry. The laser Doppler anemometry provides the capability of measuring velocities without contact and thus free of interference. Both the flow velocity of the air, for example in a wind tunnel, and the velocity of solid bodies can be measured by means of this method. The physical basis used for this is the Doppler effect. The Doppler effect is known so that it only needs to be discussed here in a fundamental way. It describes the physical phenomenon that the frequency of a moving source emitting waves is perceived with a frequency shift by an observer who is standing still. Since the velocity of the source is proportional to the frequency shift, the velocity of the source can be determined from the frequency shift. This Doppler effect represents the physical basis for the laser Doppler anemometry.
In the method known in the prior art, a volume to be measured is illuminated by a pair of light beams which cross each other, in most cases as light of a laser. The light beam is scattered at particles which are either located in the flow in any case or have been added to the flow. The scattered light, which is Doppler-shifted compared with the light beam, is registered and converted into an electric signal. This electric signal typically consists of several wave trains. To determine the flow velocity, the electric signal, which is called a burst in the technical language, must be evaluated with respect to its frequency. A fast evaluation is desirable in this context in order to keep the measuring time as short as possible. The method should also be usable with relatively high degrees of turbulence of the flow or in measurements close to the wall, that is to say when interfering scattered light (noise) occurs. To achieve this, it is known to split the electric signal of the registered scattered light into sine and cosine terms of the fundamental frequency and its harmonics with the aid of a fast Fourier transformation. The dominant frequency of the electric signal of the registered scattered light is then obtained from the component having the highest intensity. This fast Fourier transformation is carried out in a processor. The computing time needed for this is of the order of one second. Since reliable information on the flow velocity can only be obtained by averaging over several bursts, relatively long measuring times are produced which make the method unattractive. To shorten these measuring times, it is also known to carry out the fast Fourier transformation with processors specially developed for this purpose. The increase in computing speed is also achieved by the fact, among other things, that from the entire burst only a limited number of interpolation points, usually 64, from the electric signal of the registered scattered light, represented against time, is used. This is disadvantageous insofar as, as a result, reliable results can no longer be achieved if the quality of the signals is poor, that is to say with high noise. A corresponding increase in the number of interpolation points would again entail an extension of the computing time. It is also disadvantageous that such processors are relatively expensive.
It is also known to measure the zero transitions of the electric signals of the registered scattered light. So-called counters are used for this. These counters can be used for evaluating several 100,000 measurements/second which corresponds to a very fast evaluation. The disadvantageous factor in using counters is that an unambiguous counting must be possible, that is to say that the electric signal must not be noisy. Measurements close to the wall or with a high degree of turbulence cannot be evaluated with these counters. In particular, a high degree of turbulence of the flow cannot be distinguished from noise-related fluctuations in the signals.